1. Field of the Invention
The present invention relates to novel, oriented, thick composite polyester films having a variety of final application coatings improvedly adhered thereto, to a process for the production thereof, and to the final application coated films fabricated therefrom.
As utilized herein, by the term "thick polyester films" are intended films having a thickness greater than 40 micrometers.
2. Description of the Prior Art
It is known to this art that polyester films can be used for many different applications, depending upon their thickness. Thus, the so-called thin films, having a thickness of less than 40 micrometers, are employed as supports for magnetic tapes intended for recording various signals (audio, video and computer signals), or as packaging materials. The so-called thick films, on the other hand, are used as supports for a number of different applications, such as in the graphic arts, mounting supports, masking films or overhead projection films. These films typically receive an application coating which permits their ultimate use. Thus, coatings such as a magnetic film, a printing with an ink, a metal coating, a photosensitive coating and a coating for changing the permeability thereof to gases and/or to water vapor are, for example, deposited onto thin films. Thick films, for their part, are provided with a matte coating intended for drawing, for example a diazo coating. These coatings will hereinafter be referred to by the expression "final application coating".
It is also known to this art that, by nature, the bonding of the final application coatings onto the surface of polyester films is not easy. In fact, polyester films have a relatively smooth surface, limited chemical reactivity, and a very low sensitivity to solvents. Therefore, it is most often necessary to first apply thereto an "adhesion primer" coating of more or less complex chemical formulae, which enables the application coatings to be anchored. In general, this adhesion primer (or anchoring layer) is applied by the individual conducting the conversion. Therefore, it is easy to adapt the nature of the primer to the application ultimately made.
Film manufactures have themselves sought to modify the surface of films by various means. For this purpose, the use of polymers of varied nature as adhesion primers has been proposed. A particularly advantageous class of polymers employed for this purpose consists of copolyesters having a plurality of recurring hydrophilic groups and especially hydroxysulfonyl groups, or their metal salts (free sulfonic acid groups or sulfonic acid salts will hereinafter be referred to by the term "oxysulfonyl group" for convenience). Adhesion primers of this type may be deposited either by coating the support film using a dispersion or an aqueous solution (cf. French Patents Nos. 1,401,581 and 1,602,002; U.S. Pat. No. 4,476,189 and European Patent No. 78,559), or by coextrusion or rolling (cf. Japanese Patent Applications published under Nos. 50/135,086 and 79/153,883). The deposition of an adhesion primer of the oxysulfonyl group-containing copolyester type by coating involves the use of copolyesters having high contents of oxysulfonyl group-containing recurring units such as to make it possible to disperse or to dissolve the sulfonated copolyester in water. The coating process lacks flexibility because it restricts the rang of sulfonated copolyesters which can be employed to those having a high content of oxysulfonyl group-containing recurring units. Moreover, depending on their composition, such sulfonated copolyesters may have low bonding temperatures which make the composite films difficult to handle or to use.
Coextrusion is an advantageous process for applying an adhesion primer coating. The improvement in suitability for printing and the adhesiveness to metal coatings of thin crystalline or semicrystalline polyester films by depositing a sulfonated copolyester-based anchoring layer by coextrusion has been proposed in Japanese Patent Application published under No. 79/153,883. To this end, copolyesters containing from 0.5 to 50 mol % and preferably from 1 to 20 mol % of oxysulfonyl group-containing recurring units relative to recurring units of the same nature are employed. The copolyesters employed are, more particularly, polycondensates containing a plurality of terephthalate and 5 sulfoisophthalate recurring units. It was observed that the improvement in adhesiveness of crystalline or semicrystalline polyester films to their final application coatings meets different demands according to their thickness. In general, because of the nature of their final coating, thick films require sulfonated copolyesters having a higher content of oxysulfonyl group-containing recurring units than thin films; in general, the sulfonated copolyesters employed for thick films must contain more than 3 mol % of sulfonated recurring units relative to recurring units of the same nature. However, coextrusion proves to be industrially unsuitable for the production of oriented composite polyester films containing, on at least one of their face surfaces, an adhesion primer coating based on a sulfonated copolyester having a high content of sulfonated recurring units. Thus, it was observed that sulfonated copolyesters of the ethylene glycol terephthalate/5-sulfoisophthalate type having a high content of 5-sulfoisophthalate units, for example greater than or equal to 10 mol %, do not easily lend themselves to the production of composite films by coextrusion. Their use entails a lengthy and expensive drying step because of hygroscopicity; an insufficient drying interferes with the progress of the coextrusion process and results in poor quality films. Additionally, the theological properties of sulfonated copolyesters are greatly modified by the presence of recurring units containing oxysulfonyl groups and are different from those of the polyester employed for producing the support film. This difference in rheological properties of the two polycondensates complicates the production of a composite film by coextrusion, or even makes it impossible. To summarize, the manufacturer of thick, oriented, crystalline or semicrystalline polyester films must compromise between two opposing requirements: improving the adhesiveness of thick films to their final application coatings by using sulfonated copolyesters, but without interfering with the coextrusion conditions. These two requirements cannot be satisfied by the use of any sulfonated copolyester of the prior art.